It is expected that the LTE (Long Term Evolution) system will allow an increased usage of device to device communication. The LTE system and LTE-A (LTE Advanced) system comprises the Evolved Universal Mobile Telecommunications System (UMTS) Terrestrial Radio Access Network (E-UTRAN), and the Evolved Packet Core (EPC). The E-UTRAN typically comprises a plurality of base station entities, typically referred to as eNBs (evolved NodeBs) for macro-cells and HeNBs (Home-eNBs) for femto-cells, as well as the cellular terminals (or user equipments).
Device to device communication, especially for the usage for critical communication in public safety is currently being defined as a work item in the context of 3GPP (Third Generation Partnership Project), Release 12. In this context, one goal is to allow a replacement of the TETRA system, which is widely used for critical communication today, with current radio technology based on LTE (Long Term Evolution).
In order to fulfil the requirements of the public safety authorities, at least voice communication has to be supported in such a system. In a later step, all kinds of multimedia based communication should be provided for such critical communication.
The basic principle of LTE device to device communication (LTE D2D) is the possibility to allow a direct communication between at least two devices (or user equipments) supporting a device to device communication mode. Within the context of the device to device communication, standardized in 3GPP, two modes are part of the 3GPP definitions: an infrastructure-based device to device communication mode and a direct device to device communication mode.
A further feature of the device to device communication is the so-called “discovery” functionality which allows at least two device-to-device-communication-enabled devices (or user equipments) to identify each other if these devices (or user equipments) are in the vicinity of each other.
The discovery feature is of interest to the critical communication users as well as of interest to commercial cellular mobile operators in order to explore new business opportunities with targeted mobile advertising as well as supporting the growing trend of social networking where the environment of the current location of the user equipment (i.e. the proximity of the current location) plays a more and more important role. Additionally, device to device communications, and especially proximity services, are likewise interesting for automotive applications in order to establish new services, e.g., for car-to-car communication, traffic management and traffic alert.
The successful execution of the discovery procedure between two user equipments typically necessitates the use of a correct (or coherent) time base (or an indication of a common timing) of the two user equipments that are located in proximity to each other. Such a coherent information about the time base is difficult to maintain in case that one of the two user equipments is out of radio coverage of the respective base station entity of the mobile communication network, and this coherent information about the time base deteriorates the longer the one user equipment without radio network coverage is without radio coverage. This might lead to situations where no successful execution of the discovery procedure is possible between two user equipments.